Lehui Xiao

Inhibition of beta 1–40 amyloid fibrillation with N-acetyl-l-cysteine capped quantum dots

One of the primary factors that induce Alzheimers disease (AD) is the deposition of beta-amyloid (Abeta). The Abeta molecules can self-assemble to form neurotoxic aggregates with various morphologies, such as dimers, oligomers, protofibrils and fibrils. For this aspect, we demonstrated that the amyloid fibrillation can be inhibited by quenching the nucleation and elongation processes with a low concentration of water dispersed N-acetyl-L-cysteine capped quantum dots (NAC-QDs). Based on the concentration dependence of NAC-QDs on the seeded fibril growth, there is a remarkable inhibition effect when the NAC-QDs concentration is increased by 100-fold from 10(-9) to 10(-7) M. The NAC-QDs concentration required to show inhibition effect is much lower than that of the amyloid peptide concentration (50 microM). The step-like change suggests that the inhibition effect of NAC-QDs displays a threshold response. The inhibition is likely due to the intermolecular attractive interactions such as the hydrogen bonding between NAC-QDs and amyloid fibrils resulting in the blockage of the active elongation sites on the fibrils.

Three Dimensional Orientational Imaging of Nanoparticles with Darkfield Microscopy

The complete three-dimensional orientations of single gold nanorods (AuNR) were successfully resolved by using a standard optical darkfield microscope through deciphering the field distribution pattern in the slightly defocused darkfield images. The resulting images depend on the aspect ratio of the AuNR, the numerical aperture of the objective, the defocusing distance, and the polarization direction of the incident radiation. Interpretation of the observed images is facilitated by comparing them with a series of simulated images with different parameters. The experimental data matched well with the simulated results, and the reliability of this technique was further verified with polarization modulation experiments. Since deconvolution can be performed off-line after the images are recorded, this approach essentially allows video-rate data acquisition. The convenient, reliable and rapid angle-resolving capability should enable broad applications in imaging studies in many scientific fields.

Imaging Translational and Rotational Diffusion of Single Anisotropic Nanoparticles with Planar Illumination Microscopy

Here we demonstrated a simple yet powerful method, planar illumination microscopy, to directly track the rotational and translational diffusion dynamics of individual anisotropic nanoparticles in solution and living cells. By illuminating gold nanorods (GNRs) with two orthogonal sheets of light and resolving the polarized scattering signal with a birefringent crystal, we readily achieved three-dimensional angular resolving capability for single GNRs in noisy surroundings. The rotational dynamics of individual GNRs dispersed in glycerol/water mixtures with different chemical modification were tracked, and the measured rotational diffusion coefficient was well fitted to a previously reported theoretical model (Torre, J. G. d. l.; Martinez, M. C. L. Macromolecules 1987, 20, 661-666; Tirado, M. M.; Torre, J. G. d. l. J. Chem. Phys. 1980, 73, 1986-1993). In addition, the translational and rotational movements of individual GNRs transported by kinesin motor protein on microtubules inside living cells were directly imaged. Compared to its motion in free solution, a GNR attached to motor-protein did not rotate significantly while moving forward. Our method can be further generalized to allow determination of three-dimensional orientation of single dipoles using many different illumination modes.

Single Molecule Biosensing Using Color Coded Plasmon Resonant Metal Nanoparticles

Plasmonic metal nanoparticles (NPs) exhibit size, shape, and composition-dependent optical properties and have been studied extensively in the field of biosensing, but wide applications so far are limited by a lack of both highly sensitive and quantitative yet cost-effective optical detection schemes. We present a single molecule biosensing method based on color differentiation of scattered light between single plasmonic NPs and DNA hybridization-induced NP aggregates. With a seed-mediated NP growth method and a fast DNA modification method, highly stable, spectrally uniform and monodisperse Au NP (40 nm) and Au/Ag/Au composite NP (33 nm) probes were successfully prepared. Through theoretical calculations, single NP spectral measurements, and real time single-NP tracking experiments, we show that binding of a single target molecule between two NP probes can be recognized without separation from the unbound NPs by simply using a darkfield microscope equipped with a conventional light source and a color charge coupled device (CCD) camera. The detection limit of this homogeneous assay reached 0.02 pM. As an initial demonstration of multiplexed sensing at the single NP level, we used Au NPs and Au/Ag/Au composite NPs as different color probes. This scheme could be potentially applied to other areas such as multiplexed immunoassay, single cell analysis, and real time biomolecule interaction studies.

Unsynchronized Translational and Rotational Diffusion of Nanocargo on a Living Cell Membrane

A robust high-speed and high-precision single nanoparticle translational and rotational tracking method has been developed to directly monitor the interactions between transferrin-modified nanocargos (gold nanorods) and the membrane proteins prior to endocytosis. This approach shows that the translational and rotational diffusions of nanocargos on living cell membranes are unsynchronized in space and in time.

Determining the Full Three-Dimensional Orientation of Single Anisotropic Nanoparticles by Differential Interference Contrast Microscopy†

Keeping track: By combining differential interference contrast (DIC) image pattern recognition with DIC polarization anisotropy, the exact full three-dimensional angular information of individual tilted gold nanorods positioned in the focal plane of the objective lens can be readily determined. The angular rotational modes and kinetics of individual in-focus gold nanorods can thus be resolved dynamically.

Optical Imaging of Individual Plasmonic Nanoparticles in Biological Samples

Imaging of plasmonic nanoparticles (PNP) with optical microscopy has aroused considerable attention in recent years. The unique localized surface plasmon resonance (LSPR) from metal nanoparticles facilitates the transduction of a chemical or physical stimulus into optical signals in a highly efficient way. It is therefore possible to perform chemical or biological assays at the single object level with the help of standard optical microscopes. Because the source of background noise from different samples is different, distinct imaging modalities have been developed to discern the signals of interest in complex surroundings. With these convenient yet powerful techniques, great improvements in chemical and biological assays have been demonstrated, and many interesting phenomena and dynamic processes have also been elucidated. Further development and application of optical imaging methods for plasmonic probes should lead to many exciting results in chemistry and biology in the future.

Effect of surface-functionalized nanoparticles on the elongation phase of beta-amyloid (1-40) fibrillogenesis.

The influence of nanoparticles of various sizes and surface functionalities on the self-assembling fibrillogenesis of beta-amyloid (1-40) peptide was investigated. Functionalized nanoparticles including quantum dots and gold nanoparticles were co-incubated with monomeric Aβ(1-40) peptides under seed-mediated growth method to study their influences on the elongation phase of the fibrillogenesis. It is observed that charge-to-surface area ratio of the nanoparticles and the functional moiety and electrostatic charges of the conjugated ligands on the particle surfaces took crucial regulatory role in the Aβ(1-40) fibrillogenesis.

Single Particle Dynamic Imaging and Fe3+ Sensing with Bright Carbon Dots Derived from Bovine Serum Albumin Proteins.

In this work, we demonstrated a convenient and green strategy for the synthesis of highly luminescent and water-soluble carbon dots (Cdots) by carbonizing carbon precursors, i.e., Bovine serum albumin (BSA) nanoparticles, in water solution. Without post surface modification, the as-synthesized Cdots exhibit fluorescence quantum yield (Q.Y.) as high as 34.8% and display superior colloidal stability not only in concentrated salt solutions (e.g. 2 M KCl) but also in a wide range of pH solutions. According to the FT-IR measurements, the Cdots contain many carboxyl groups, providing a versatile route for further chemical and biological functionalization. Through conjugation of Cdots with the transacting activator of transcription (TAT) peptide (a kind of cell penetration peptide (CPP)) derived from human immunodeficiency virus (HIV), it is possible to directly monitor the dynamic interactions of CPP with living cell membrane at single particle level. Furthermore, these Cdots also exhibit a dosage-dependent selectivity toward Fe3+ among other metal ions, including K+, Na+, Mg2+, Hg2+, Co2+, Cu2+, Pb2+ and Al3+. We believed that the Cdots prepared by this strategy would display promising applications in various areas, including analytical chemistry, nanomedicine, biochemistry and so on.

Noise-Free Dual-Wavelength Difference Imaging of Plasmonic Resonant Nanoparticles in Living Cells

Herein, we demonstrated a new optical microscopy method to selectively image small-size gold nanoparticles (GNPs) inside noisy living cells through determination of the difference image between the probe beam (illuminated at the resonance wavelength of GNPs, 532 nm) and the reference beam (illuminated at 473 nm). From computer simulation and single-particle imaging experiments, we demonstrated that GNPs with a diameter of 45 nm could be selectively imaged in the GNPs/cell lysates mixture and inside living cells by dual-wavelength difference (DWD) imaging. The diffusion dynamics of nucleic acids functionalized GNPs on cell membranes and the internalization kinetics of these GNPs by living cells were explored with this method. Our real-time tracking experiments showed that statistically 80% of GNPs were under restricted diffusion on the cell membrane. The cell cytoskeleton fence effect, as observed in the single-particle tracking experiments, may be one of the main factors for the restricted diffusion mode.